These sprites are licensed under the CC BY 3.0 license: http://creativecommons.org/licenses/by/3.0/. We require attribution to Gamedevtuts+, and a link to this post: http://gamedev.tutsplus.com/articles/news/enjoy-these-totally-free-space-based-shoot-em-up-sprites/, from anyone that redistributes the artwork, but we do not require attribution if you use the art in your games (regardless of whether it is for commercial or personal use, or for a prototype or final product).